A SOFC is an electrochemical device for the generation of electrical energy through the electrochemical oxidation of a fuel gas (usually hydrogen-based). The device is generally ceramic-based, using an oxygen-ion conducting metal-oxide derived ceramic as its electrolyte. As most ceramic oxygen ion conductors (for instance, doped zirconium oxide or doped cerium oxide) only demonstrate technologically relevant ion conductivities at temperatures in excess of 500° C. (for cerium-oxide based electrolytes) or 600° C. (for zirconium oxide based ceramics), SOFCs operate at elevated temperatures.
In common with other fuel cells, SOFCs include an anode where fuel is oxidised, and a cathode where oxygen is reduced. These electrodes must be capable of catalysing the electrochemical reactions, be stable in their respective atmospheres at the temperature of operation (reducing on the anode side, oxidising on the cathode side), and be able to conduct electrons so the electric current generated by the electrochemical reactions can be drawn away from the electrode-electrolyte interface.
Finding materials with the relevant combination of properties for the anode has, in spite of extensive research, proved difficult. For many years, the state-of-the-art SOFC anode has consisted of a porous ceramic-metal (cermet) composite structure, with nickel as the metallic phase and an electrolyte material (usually yttria or Scandia-stabilised zirconia) as the ceramic phase, although less commonly doped ceria-based electrolyte materials such as gadolinia or samaria-doped ceria have also been used. In this structure, the nickel performs the role of catalyst, and the volume fraction of nickel is high enough that a contiguous metal network is formed, thus providing the required electronic conductivity. The electrolyte material forms a contiguous ceramic backbone to the anode, providing mechanical structure, enhancing the bond between the anode and the electrolyte and also extending the anode-electrolyte interfacial region some distance into the anode.
A well-known limitation of these cermet anodes is that at cell operating temperature the metallic nickel in the anode is only stable in a reducing atmosphere. This is normally provided by the fuel gases, so under normal operation the anode is stable. However, should the supply of fuel gas be interrupted with the SOFC at operating temperature, the atmosphere within the anode will become oxidising. Under these conditions the metallic nickel will oxidise back to nickel oxide. This oxidation is associated with a volume increase of greater than approximately 40%, because the metallic nickel which has been formed by the reduction of sintered nickel oxide does not oxidise back to the same morphology as the original nickel oxide from which it was formed. Instead it generates mesoporosity, occupying a larger volume than the original nickel oxide. This volume change on reoxidation can generate large stresses in the anode structure, which in turn can result in cracking of the anode and potential destruction of the SOFC cell.
The inability of many SOFC cells to undergo multiple reduction-oxidation (REDOX) cycles without suffering damage of this type has been a major factor inhibiting the widespread commercial adoption of SOFC technology for power generation, as SOFC systems generally require the presence of complex and expensive purge gas systems to maintain a reducing atmosphere over the anodes in the event of an unexpected fuel interruption, for example due to a failure elsewhere in the system which requires an emergency shutdown of the system for safety reasons.
The problem of inadequate REDOX stability is particularly acute in anode supported fuel cells, currently the most common form of SOFC cell. Anode support is beneficial as it allows a very thin (<20 μm) layer of electrolyte (such as stabilised zirconia) to be used, as the electrolyte is non-structural. This in turn allows operation at a lower temperature range than is the case for electrolyte supported cells (650 to 800° C. rather than 850 to 1000° C.). Because the resistance of the electrolyte to oxygen ion transport is inversely proportional to the electrolyte thickness, in electrolyte supported fuel cells, the resistance caused by the thickness of the electrolyte layer is overcome by increasing operation temperatures, exploiting the exponential drop off in resistance with temperature. As thinner layers can be used in anode supported cells, operation temperatures can be reduced, which is generally desirable as it facilitates the use of lower-cost materials in the SOFC system, and reduces the rate of various material degradation mechanisms such as the oxidation of metallic components.
In spite of these advantages, as the anode is the structural support of the SOFC cell in an anode-supported cell, the cells are very prone to catastrophic failure on repeated REDOX cycling, as stress-induced cracking can result in the cell completely breaking up.
In spite of considerable efforts by developers, no alternative to nickel has achieved widespread adoption, as no suitable material has yet been developed which combines nickel's relatively low cost, high catalytic activity for both electrochemical oxidation of hydrogen and steam reforming of hydrocarbon fuel feeds, and high electronic conductivity.
Gorte et. al. (US 2005/227133 A1, U.S. Pat. No. 7,014,942 B2) have reported the use of copper in a SOFC anode partially or completely substituted for nickel. Copper has advantages as an electronically conductive phase in the anode, notably that it does not catalyse the formation of carbon from hydrocarbon fuels. However it is a poor catalyst for the electrochemical oxidation of hydrogen and steam reforming of hydrocarbon fuels, so in the copper anodes tested by Gorte et al., an additional catalyst such as ceria was required to achieve adequate electrode performance. The other issue with the use of copper in conventional SOFC applications is that both copper metal and copper oxide have low melting points (1084° C. and 1326° C. respectively). Cermet anodes are typically formed by sintering a mixture of the metal oxide powder and the electrolyte powder at 1200-1500° C. in air, followed by reduction of the metal oxide to the metal using hydrogen on first operation of the SOFC. This range of sintering temperatures is either close to or above the melting point of copper oxide (nickel oxide by contrast melts at 1955° C.), leading to excessive sintering of the copper oxide phase. Also conventional SOFC operating temperatures are in the range 700-900° C., close to the melting point of metallic copper, which tends to result in sintering of the copper phase during SOFC operation, potentially causing performance degradation. To address this issue, Gorte et al. developed a method of adding the copper to the anode in a post-sintering infiltration step using solutions of copper salts which were dried and then calcined to decompose the salt to copper oxide, thereby avoiding the need to sinter copper oxide at high temperatures. However, the infiltration step, whilst allowing the use of copper cermets, may be difficult to scale up to industrial production. Another issue with copper is that although less reactive than nickel, it will still oxidise if exposed to an oxidising atmosphere at temperature, and thus a copper-based anode also lacks REDOX stability.
There are factors relating to the design of the SOFC which can help mitigate the damaging effects of REDOX cycling, these include:                Not using an anode supported cell—the anode can therefore be thinner; reducing the overall volume change through REDOX cycling and the danger of catastrophic cracking.        Operating at a lower temperature—the rate of nickel oxidation increases exponentially with increasing temperature, starting at >300° C. The lower the temperature of operation, the less risk of nickel oxidation and volume expansion. Further, nickel particles tend to oxidise though a core-and-shell mechanism, where the outer surface oxidises rapidly, but then the core of the particle oxidises more slowly as this is diffusion limited. Thus at lower temperatures, it is likely that only the outer surface of the nickel particles in the anode will reoxidise, not the entire particle and any volume change will be reduced.        Provide the anode with a contiguous ceramic ‘backbone’—As the electrolyte-based ceramic phase used in SOFC anodes is largely unaffected by changes in oxygen partial pressure, this part of the anode will not change volume during REDOX cycles affecting the nickel phase. Thus the structural integrity of the anode and its bond to the electrolyte will be enhanced if there is a sintered porous ceramic network within the anode.        
A design of SOFC cell which has the potential to meet these criteria is the metal-supported SOFC design disclosed by the applicant in GB 2 368 450. This SOFC cell uses a ferritic stainless steel foil as a structural support. The foil being made porous in its central region to allow fuel access to the anode. The active cell layers (anode, electrolyte and cathode) are all deposited on top of the substrate foil as films. This means the anode only needs to be around 15 μm thick as it is not the structural support for the cell. This cell also allows operation at temperatures in the range 450-650° C., much lower than standard operating temperatures. This is achieved through the use of predominantly cerium oxide (ceria)-based ceramic materials such as CGO10 (gadolinium doped-cerium oxide, for CGO 10-Ce0.9Gd0.1O1.95) as the oxygen ion conducting electrolyte, which have an intrinsically higher oxygen ion conductivity than zirconia-based materials. A thin film of stabilised zirconia is deposited in the electrolyte to prevent internal short-circuiting of the cell due to the mixed ionic-electronic conductivity of ceria-based electrolytes, as disclosed in GB 2 456 445, but as the zirconia layer is so thin, its resistance to oxygen ion transport is sufficiently low that low-temperature operation is not prevented. The SOFC cell of GB 2 368 450 uses a porous metal-CGO10 composite cermet anode fabricated as a thick film with a thickness between 5 and 30 μm. The anode is generally deposited by screen-printing an ink containing metal oxide and CGO10 powders and formed into a porous ceramic layer by thermal processing to sinter the deposited powders together to form a contiguous structure bonded to the steel substrate.
A limitation imposed by the deposition of the ceramic layers onto a ferritic stainless steel support by conventional ceramic processing methods is the maximum temperature to which the steel may be exposed in an oxidising atmosphere due to the formation of a chromium oxide scale at high temperatures in an oxidising atmosphere. This upper limit is substantially below the 1200-1500° C. typically used when sintering ceramics and so methods have been developed for sintering rare earth doped ceria electrolytes to >96% of theoretical density at <1100° C., facilitating the formation of the gas-tight layer desired (GB 2 368 450, GB 2 386 126 and GB 2 400 486).
Surprisingly, sintering a nickel oxide-rare earth doped ceria composite anode at these temperatures has proved more difficult than sintering the electrolyte. This is because composites of two different oxide materials have been found to sinter more poorly than a single phase material. Thus nickel oxide or the ceramic alone will sinter adequately at these temperatures, but as a composite sintering in air can be poor, leading to weak necks between particles and a weak ceramic structure. This can result in cell failure as a result of REDOX cycling, as the weak bonds between nickel particles break as a result of the volume changes during the REDOX cycle. This can ultimately result in the catastrophic failure of the cell through delamination of the electrolyte from the anode.
In order to improve the REDOX stability of the cell, it is desirable to find a means of enabling sufficient sintering of the cermet structure at the temperature range at which it is possible to fire the ceramic layers on a steel substrate. It would therefore be advantageous to provide for a method of preparing a metal-supported SOFC in which the anode is stable to redox cycling, robust to a loss of reducing atmosphere at operating temperature, and yet can be made using commercially viable production methods. The invention is intended to overcome or ameliorate at least some aspects of this problem and those described above.